Phase equilibria in Mg(NO3)2–Ln(NO3)3 (Ln = Nd, Sm, Eu, Tb)–HNO3 (∼20%)–H2O systems at 298.15 K and thermodynamic properties of Mg3Ln2(NO3)12·24H2O (Ln = Nd, Sm)

Highlights: • Solubility was measured for Mg(NO₃)₂–Ln(NO₃)₃ (Ln = Nd, Sm, Eu, Tb)–HNO₃ (∼20%)–H₂O systems at T = 298.15 K. • The corresponding phase diagrams were plotted. • Two new solid phase compounds Mg₃Ln₂(NO₃)₁₂·24H₂O (Ln = Nd, Sm) were characterized. • Δ_f$H_{\text{m}}^{\Theta }$for two compounds were determined. -- Abstract: The phase diagrams of the Mg(NO₃)₂–Ln(NO₃)₃ (Ln = Nd, Sm, Eu, Tb)–HNO₃–H₂O systems in the (∼20%) HNO₃ regions at T = 298.15 K were determined by using an isothermal solution saturation method. The Schreinemaker's wet residues method was used to determine the compositions of solid-phases. Both Mg(NO₃)₂–Nd(NO₃)₃–HNO₃–H₂O and Mg(NO₃)₂–Sm(NO₃)₃–HNO₃–H₂O systems are complicated with three equilibrium solid phases Mg(NO₃)₂·6H₂O, Mg₃Ln₂(NO₃)₁₂·24H₂O and Ln(NO₃)₃·6H₂O (Ln = Nd, Sm). But the systems Mg(NO₃)₂–Eu(NO₃)₃–HNO₃–H₂O and Mg(NO₃)₂–Tb(NO₃)₃Tb–HNO₃–H₂O are simple eutonic type. The two new solid-phase compounds Mg₃Nd₂(NO₃)₁₂·24H₂O and Mg₃Sm₂(NO₃)₁₂·24H₂O are congruently soluble in an average medium of ∼20 mass % HNO₃, and they were identified and characterized by the methods of powder X-ray diffraction, X–ray diffraction single crystal structure analysis, and thermogravimetric/differential thermogravimetric. The XRD diffraction patterns for the two new solid-phase compounds are identical due to their isomorphic structure. They belong to hexagonal systems, with space group R₃. For Mg₃Nd₂(NO₃)₁₂·24H₂O, a = b = (1.0993 ± 0.0005) nm, c = (1.7211 ± 0.0007) nm, V = (1.8012 ± 0.0013) nm³, D_c = (4.2532 ± 0.0024) g·cm⁻³, Z = 3; for Mg₃Sm₂(NO₃)₁₂·24H₂O, (a = b = 1.1198 ± 0.0004) nm, c = (1.7257 ± 0.0006) nm, V = (1.8742 ± 0.0011) nm³, D_c = (4.1201 ± 0.0012) g cm⁻³, Z = 3. The standard molar enthalpies of solution of the compounds Mg₃Nd₂(NO₃)₁₂·24H₂O and Mg₃Sm₂(NO₃)₁₂·24H₂O in water (aq) were measured to be (101.26 ± 0.77) kJ mol⁻¹ and (80.27 ± 0.81) kJ mol⁻¹, and their standard molar enthalpies of formation were calculated to be – (12214.13 ± 2.5) kJ mol⁻¹ and – (12183.6 ± 2.5) kJ mol⁻¹, respectively.